1. Technical Field
The present invention relates to a method and system for servicing a wireless communication network in general, and in particular to a method and system for servicing a mobile telephone communication network. Still more particularly, the present invention relates to a method and system for managing forward link power control within a code-division multiple access mobile telephone communication network.
2. Description of the Prior Art
A code-division multiple access (CDMA) communication system is a spread-spectrum communication system in which multiple mobile telephones (mobiles) have access to the same frequency band. If each individual mobile within a CDMA communication system is allowed to have a different received signal strength, near-far interference may occur. In other words, the signal from a near mobile may interfere with the signal from a mobile that is far away. Such near-far interference is detrimental to the quality of a CDMA communication system and can degrade performance, reduce capacity, and induce call drops. Hence, the transmitting power of each mobile has to be controlled so that the received power at a cell is constant at a predetermined level, regardless of the distance between mobiles and the base station. Typically, a mobile power control unit is utilized to produce a nominal received power from all mobiles within a given cell or sector. According to the North American IS-95 standard, CDMA power control is a three step process: (1) reverse link open-loop power control; (2) reverse link closed-loop power control; and (3) forward link power control.
Reverse link open-loop power control is accomplished by adjusting the transmit power of a mobile such that the received signal at a base station is constant regardless of the mobile distance. Each mobile computes the relative path loss and compensates the path loss by self-adjusting its transmitting power. The total received power at the cell site is the sum of all powers, which determines the system reverse link capacity.
Reverse link closed-loop power control is accomplished by means of a power up or power down command originating from the cell site. Every 1.25 ms, a single power control bit (e.g., a "1" for power down by 0.5 dB and a "0" for power up by 0.5 dB) is inserted into a forward encoded data stream. Upon receiving this command from the base station, the mobile responds by adjusting the power by an amount of .+-.0.5 dB.
The goal of forward link power control is to allocate each forward traffic channel with just enough digital gain to close the forward link; that is, to provide the mobile with just enough signal power to meet the required E.sub.b /N.sub.o (i.e., signal-to-noise requirement) for the mobile. Needless to say, allocating too little digital gain will result in a degraded or dropped call, but allocating too much digital gain will result in a reduction of sector capacity. Because the digital gain needed to close the forward link varies with changes in channel conditions, changes in mobile position as well as speed, and changes in soft/softer handoff status, the allocation of forward traffic channel gain must be dynamic.
In order to protect against allocating a mobile too much digital gain, a linear-decay algorithm is currently employed for performing forward link power control. This linear-decay algorithm, however, may also provide a higher than necessary average digital gain when the mobile's required E.sub.b /N.sub.o does not change. Because the forward link capacity is inversely proportional to the square of the average digital gain, it is desirable to provide an improved method for managing forward link power control such that the digital gain of a mobile is minimized while maintaining the target frame error rate (FER) for the mobile.